U.S. Pat. Nos. 4,943,323, 5,017,234, and 5,084,103 and EP 0415799B1 of Gartner et al. disclosed the use of triisopropanolamine (hereinafter “TIPA”) for enhancing the late day strength (e.g., at 7- and 28-days) of cement and blended cement. This additive could be admixed with cement powder or interground as an addition with cement clinker during finish milling.
The term “cement” is used to refer to a binder material that, when mixed with water, forms a paste that hardens slowly to form rock-hard products such as mortar or concrete. Portland cement is distinguished from other cements by the different components of which it is composed, and the requirement that it meet particular standard specifications established in each country. For example, in the United States, the American Society for Testing and Materials (ASTM), American Association of State Highway and Transportation Officials, and other government agencies have set certain basic standards for cement which are based on principal chemical composition requirements of the clinker and principal physical property requirements of the final cement mix.
For purposes of this invention, the term Portland cement is intended to include all cementitious compositions meeting the requirements of the ASTM (as designated by ASTM Specification C150). Portland cement is prepared by sintering a mixture of components including calcium carbonate (as limestone), aluminum silicate (as clay or shale), silicon dioxide (as sand) and miscellaneous iron oxides. During sintering, chemical reactions take place wherein hardened nodules, commonly called clinkers, are formed. Portland cement clinker is formed by the reaction of calcium oxide with acidic components to give primarily tricalcium silicate, dicalcium silicate, tricalcium aluminate, and a ferrite solid solution phase approximating tetracalcium aluminoferrite (C4AF).
After the clinker has cooled, it is then pulverized together with a small amount of gypsum (calcium sulfate) in a finish grinding mill to provide a fine, homogeneous powdery product known as Portland cement.
The term “blended cements” typically refers to a combination of Portland cement and secondary cementitious materials. Because of the rigid compositional and physical requirements for forming suitable Portland cement clinker, such cement clinker becomes a relatively expensive raw material. Thus, for certain applications, it is possible to substitute a portion of the Portland cement with secondary cementitious materials such as less expensive fillers or clinker substitutes including limestone, ground granulated blast furnace slag, fly ash, natural or artificial pozzolan, and the like.
As used herein, the term filler typically is used to refer to an inert material that has no later age strength enhancing attributes.
The term “clinker substitute” refers to a material that may contribute to long term compressive strength enhancement, but usually exhibits little or no enhancement of 7- or 28-day compressive strength. The addition of these fillers or clinker substitutes to form “blended cements” is limited in practice by the fact that such addition usually results in a diminution in the physical strength properties of the resultant cement. For example, when filler material such as limestone is blended with cement in amounts greater than 5%, the resultant cement exhibits a marked reduction in strength, particularly with respect to the strength attained after 28 days of moist curing (28-day strength). The 28-day strength has particular significance and will be emphasized throughout this invention since it is the strength at this age which is most commonly used to assess the engineering properties of the final cement products.
It was observed by Gartner et al. that the addition of triisopropanolamine (“TIPA”) to cement or blended cement, while enhancing the later age strength of the cement, also tended to increase the amount of air entrained in the cement (U.S. Pat. No. 4,943,323, column 6, lines 34 et seq.). Analysis of various cement samples revealed an increase in air entrainment of about 2% when compared to cement that did not contain TIPA. Hence, the use of an air detraining agent or “air detrainer” was proposed for eliminating the increased air entrainment in the cement due to the presence of TIPA. Gartner et al. suggested that the air detrainer should be compatible with TIPA in that the detrainer should not degrade TIPA and that TIPA be soluble therein or made soluble by addition of further ingredients.
Gartner et al. therefore described suitable air detraining agents to include nonionic surfactants such as phosphates, including tributylphosphate, phthalates including diisodecylphthalate, and block copolymers including polyoxypropylene-polyoxyethylene-block copolymers (U.S. Pat. No. 4,943,323, Column 6, lines 50-55). For the purpose of withstanding the heat generated by the grinding of clinker in a cement grinding mill, the inventors preferred using nonionic polyoxypropylene-polyoxy-ethylene block copolymers having molecular weight of at least 2500 (U.S. Pat. No. 4,943,323, Col. 6, lines 62-66).
In U.S. Pat. No. 5,156,679, Gartner et al. taught the use of water-soluble alkylated alkanolamine salts for detraining air in hydraulic cement structures, and in particular concretes. Added as admixtures to cement, these materials included N-alkylalkanolamine and N-alkyl-hydroxylamine. In example 1, Gartner et al. demonstrated that when TIPA was added to a mortar mix in the amount of 0.02% by weight as part of the water of hydration along with 0.01% by weight of dibutylamino-2-butanol (“DBAB”) as defoaming agent, the mortar mix demonstrated a reduction in air entrainment (Col. 5, line 51-Col. 6, line 14).
While the above-mentioned air detrainers may be suitable for detraining air when incorporated directly into cement mortar, the heat and humidity of the grinding mill environment, coupled with the alkaline condition of the cement and the grinding-shearing forces imposed by the mechanical process of the grinding, tend to degrade the molecular structure of the defoamer, sometimes to the point at which it begins to entrain air rather than to detrain it.
U.S. Pat. No. 5,429,675 of Cheung et al. disclosed grinding aid compositions for grinding clinker into hydraulic cement powder, wherein the grinding aid comprised a mixture of at least one alkylene glycol and particulate carbon in a weight ratio of 1:0.1 to 1:0.5. The grinding aid composition could optionally contain alkanolamines with at least one C2-C3 hydroxyalkyl group.
When concrete is formed, it requires mixing of the various components such as hydraulic cement, sand, gravel, water, and possibly chemical additives and/or admixtures to form a substantially uniform mixture. In the course of mixing, air becomes entrapped in the composition and much of this air remains in the resultant cured composition in the form of air voids. If void size is small, the mix is said to be “air entrained.” In most instances, a small amount of air entrainment is tolerated and, in certain instances, it is desired to enhance durability to freeze/thaw cycles in the environment. However, air entrainment in the hydraulic cement composition is not a desirable feature as it causes the resultant structure to have lower compressive strength than the mixture design is capable of attaining. There is an inverse relationship between air entrainment and compressive strength. It is generally believed that for each volume percent of air voids (bubbles) in a concrete mass, there exists a five percent loss of compressive strength.
Various materials are presently used in the concrete industry to reduce the amount of air contained in cured hydraulic cement compositions. Conventional air-detraining agents are generally viewed as surfactants having low hydrophilic-lipophilic balance (HLB) values, such as tri-n-butylphosphate, n-octanol and the like. Normally, these agents have been found difficult and somewhat ineffective to use in commercial applications for several reasons. Firstly, they cannot be readily introduced into dry concrete mixes due to the difficulty in dispersing the additive throughout the cement to provide a uniform distribution of the small amount of agent required. Further, the conventional air detrainers are not miscible with and, therefore, not capable of being added with other conventional cement admixtures as such admixtures are invariably water-based compositions. When it is attempted to incorporate an air-detrainer into an aqueous admixture composition, it tends to separate out and is not properly supplied to the cement composition to be treated. “Water-dispersible” air-detrainers were introduced in an attempt to overcome this problem. These agents still have low HLB values and are actually not water soluble but merely have densities close to that of water. These agents phase-segregate and are unstable in aqueous suspension in storage and, thus, have the same defects of prior known air-detrainers.
Air-detraining agents are generally very powerful in their effectiveness and, therefore, must be used in very small amounts which must be substantially uniformly distributed throughout the cement composition being treated. Presently known air-detraining agents have the disadvantages of being difficult to monitor and control in terms of dosage and distribution in cement compositions, thus causing the composition to exhibit unwanted variation from the desired degree of aeration (due to over or under dosage) and/or variation in aeration within the formed structure (due to poor distribution of agent).
Thus, a novel composition and method are required for enhancing early and late strength (i.e., early strength=1-3 days after water is added to the cement to initiate hydration; late strength=7- and 28-days after water is added to the cement to initiate hydration) in cements using at least one air-entraining cement additive, of which TIPA is an example, while also achieving robust air detrainment that survives the harsh cement milling conditions and enables strength of the mortar to be preserved.